Issue 78

A. Bacco et alii, Fracture and Structural Integrity, 78 (2026) 1-18; DOI: 10.3221/IGF-ESIS.78.01

The macrographic analysis served a dual purpose: it confirmed the absence of micro-defects undetected by radiography and, crucially, provided the experimental geometrical dimensions of the FZ and HAZ. These measurements were fundamental to calibrate the shape of the heat source in the numerical model.

Parameters Voltage [kV] Current [mA]

Values

181.0

2.5

Focus-Detector distance [mm] Focus-Object distance [mm]

1260.0

908.0

Pixel size [µm]

179

Gain [1pF]

2

Exposure [fps] 18 Table 4: Parameters of radiographic test.

Finally, a Vickers hardness profile (HV10) was measured across the joint cross-section. The indentations were spaced at 1 mm intervals along a line transverse to the welding direction, centred on the WCL, allowing for a detailed mapping of the mechanical property variations from the steel to the cast iron side.

FE MODEL

A

sequentially coupled thermal-mechanical analysis was adopted to predict the residual stress field. This approach assumes that the mechanical deformation has a negligible effect on the thermal history, significantly reducing computational cost without compromising accuracy. This methodology is consistent with previous numerical investigations conducted by the authors on similar joint configurations [18]. All simulations were performed using the commercial F code Abaqus/CAE 6.14-1. To ensure consistency between the thermal and mechanical solutions, an identical mesh topology was employed for both analyses, differing only in the element type: 8-node linear heat transfer brick elements (DC3D8) were used for the thermal solver, while 8-node linear brick elements (C3D8) were selected for the structural analysis. The mesh discretization was refined in the FZ and HAZ to capture the steep thermal gradients. To accurately simulate the thermo-mechanical behaviour of the joint, the physical and mechanical properties of the materials involved were modelled as temperature-dependent functions. Since experimental characterization at high weld temperatures was not performed in this study and since these are alloys widely used in industry (not special alloys), the non-linear material properties (both thermal and mechanical) were derived from consolidated literature data. Specifically, the thermo mechanical properties (conductivity, specific heat, density) (Fig. 3) for the S355 steel were adopted from the study by [19], while for the spheroidal cast iron, reference was made to the work of [12] on the influence of temperature on ductile iron. Regarding the elasto-plastic behaviour, temperature-dependent stress-strain curves were implemented based on experimental data reported by [20] for the steel component and by [12,21] for the nodular cast iron. As for the filler material, due to dilution with the base metals and the lack of specific data for the resulting alloy, the properties were assumed to be consistent with those of the spheroidal cast iron, a simplified approach successfully adopted in previous computational studies [12], in which the wire is simulated using the properties of the base material. This choice avoids steel because the massive carbon dilution from the cast iron makes the weld pool metallurgically closer to it. Furthermore, Ni-Fe fillers are engineered to match the thermal expansion of cast iron to minimize shrinkage stresses; modelling them this way avoids spurious stress peaks in the brittle cast iron HAZ, safely shifting the thermo-mechanical mismatch to the ductile and tough steel side. To simulate the progressive deposition of the filler material, the 'element birth and death' technique (Model Change option) was implemented. This modelling strategy, extensively tested by different research group to evaluate welding efficiency and distortions [22], involves discretizing the weld bead into multiple longitudinal sets, initially deactivated and progressively reactivated in synchronization with the moving heat source. The simulation steps replicated the experimental procedure: 1. deposition of the first pass;

2. inter-pass cooling time of 120 s; 3. deposition of the second pass; 4. final cooling to room temperature.

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